1. Field of the Invention
This invention relates to photochromic materials and more particularly, to novel photochromic spiropyran compounds.
2. Description of the Prior Art
Photochromic materials are known as those materials which undergo reversible changes in color. One of typical photochromic materials includes spiropyran compounds. A number of spiropyran compounds have been studied and developed up to now. For instance, when a spiropyran compound of the following formula (A) is irradiated with UV light with a wavelength of 340 nm, it is changed into the merocyanine of the formula (B) assuming a red color. When visible light with a wavelength of 580 nm is again applied to the merocyanine, it is returned to the spiropyran (A). ##STR1##
This property of the photochromic materials can be utilized to fabricate optical storage media. In order to miniaturize the medium, it is desirable to use a semiconductor laser as a light source for the storage media. To this end, it is necessary that photochromic materials have an absorption sensitivity in the vicinity of 700 nm which is in an oscillation region of the semiconductor laser.